In order to reduce Green House Gas (GHG) emission, especially at coal fired power plants and heavy industry such as the metallurgy industry, it is possible to generate sustainable energy by co-firing biomass in coal fired furnaces to reduce GHG. However, due to the most coal fired power plants based on pulverized coal furnaces, biomass cannot be co-fired in higher mix percentage in coal-fired power plants without pretreatment because biomass still has fiber structure in its property thus it is not easy to be grinded and pulverized. On the other hand, the energy value of biomass is much lower than coal so a pretreatment of biomass is mandatory to increase the energy density as well as the grindability in order to use the same equipment infrastructure of coal fired power plants. In addition, the wide range of types of biomass from woody base to herbaceous base, to aquatic base prevents a large-scale standardization of biomass, thus a creation of a global market of biomass fuels is difficult to realize. In order to realize a Multiple Input Single Output (MISO) concept of different biomass inputs but only one standard biomass output, densification and pretreatment of biomass are necessary to reach a national and international standardization.
Torrefaction is an incomplete pyrolysis process (FIG. 1) where biomass is under thermo-chemical pretreatment in an oxygen free or low oxygen environment. The final product of torrefied biomass is hydrophobic, has higher energy density and is easy grind-able, matching closer to the coal properties so that co-firing of torrefied biomass in coal fired power plants is much easier, requiring no costly modification or addition of existing equipment infrastructure in coal fired power plants.
On the other hand, torrefied biomass is not easy to be densified in the form of pellets or briquettes using resident lignin acting as a glue because the required high torrefaction temperature reduces the concentration of lignin in torrefied biomass and further increases the glass transition temperature of the resident lignin so that conventional pellet press would have to be operated at higher temperature beyond current technological limits and also at higher energy consumption. In order to facilitate the process of densification for torrefied biomass, external binder is generally added. Unfortunately external binders are usually not hydrophobic so that torrefied pellets or briquettes with additional binders absorb water and de-integrate by themselves when moisture content is high so that it poses logistic and storage problem, especially in rain or snow seasons.
It is known in the prior art that the feedstock needs to be dried completely before torrefaction process could start.
U.S. Pat. No. 9,347,011 teaches a torrefaction system with two separate torrefaction treatment devices of different process technologies wherein the first treatment device is mainly to dry the wet non-densified biomass while the second treatment device performs the torrefaction. The first treatment device is of a fluidized bed reactor type which has a limited feedstock flexibility and is optimized to process mainly woody biomass such as sawdust. On the other hand, the feedstock for the said torrefaction system has to be non-densified biomass because densified biomass in the form of pellets or briquettes is not suitable for fluidized bed reactor. Further densification after torrefaction in the form of pellets or briquettes is energy consuming and may require additional binders which in general are not hydrophobic.
U.S. Pat. No. 9,206,368 teaches a mass flow torrefaction reactor of one single treatment stage. The downside of this conventional reactor type is the “tunneling effect”, especially when upscaling to a large size reactor for higher mass production throughput, where hot gas coming from the bottom of the reactor chamber may find some short cuts through the biomass to the closest gas discharge outlet and thus creating different hot and cold zones within the reactor chamber leading to non-uniform quality of torrefied biomass. On the other hand, the control of torrefaction parameters of temperature, oxygen content and residence time for one single treatment stage are not flexible enough to deal with different varieties of biomass associated with different properties and moisture contents so the MISO concept is almost impossible to be realized. Furthermore, the feedstock for the said torrefaction system is non-uniform and non-densified biomass “having 25% or less moisture content and with a size of from about 13 mm to about −75 mm in the longest dimension” so that densification after torrefaction in form of uniform pellets or briquettes is energy consuming and may require additional binders which in general are not hydrophobic.
Therefore, it is an unmet need to improve torrefaction systems of the prior arts with the present invention.